We are frequently told that the Sun can’t be responsible for late C20th warming because temperature has increased while solar activity has dropped from it’s peak in the 1950’s.
What a load of rubbish.
Solar cycle amplitudes are only part of the story. The cycles in the late C20th were short, ~10 years, and high compared to the long term average of ~40 SSN. The minima between them were short too. So although they did reduce in absolute amplitude after the ’50s, they made up for it by kicking out more energy more of the time. Last year to get a handle on this, I integrated the total sunspot areas as a running cumulative total departing from the long term average.
The Sea Surface Temperature graph from woodfortrees.org with the trend lines added shows how well the sunspot cumulative total works as a proxy for Ocean Heat Content. The SST data is smoothed over 1/3 of the solar cycle length to bring out the solar effect on SST’s. I further developed this idea in my simple solar-planetary energy model. Looking at the flattening of the rise at cycle 19-20 (1954-1976) and from cycle 22-23 and now the low cycle 24, I would say we are just over the top of the warming curve. The slightly falling OHC data from 2003 onwards measured by ARGO would seem to back this up.
So although we have yet to understand all the mechanisms by which the Sun’s energies get transferred into Earth’s climate system, we can say that the solar data fits the temperature record better than co2 data does, over a longer period too.